∵ quadrilateral ABCD is a rectangle.
∴∠ABC=90
Similarly, the diagonal of the rectangular ABCD is the diameter of circle 0.
∴AC=50cm
∴△ABC is Rt△
Derived from Pythagorean theorem
AB? +BC? =AC?
∴BC? =AC? -AB?
That's BC? =2500-x?
∴ y = 2500-x under the root sign?